1. Field of the Invention
The present invention relates to a tentering oven used to manufacture biaxially stretched resin films such as biaxially stretched polypropylene (BOPP) films, biaxially stretched polystyrene (BOPS) films and synthetic papers. The tentering oven includes a stretching hot air oven (including an annealing hot air oven), a cooling furnace and a tenter, and is capable of producing stretched films having a satisfactory thickness profile in a transverse direction, while ensuring that the oil used to lubricate the chains in the tenter and which passes through a joint between rail members to flow down under gravity will not foul the film being stretched.
2. Discussion of the Background
A thermoplastic resin film containing a fine inorganic powder is first stretched in a machine direction using the difference in peripheral speed among rolls; subsequently, at least one side of the longitudinally stretched film is melt-laminated with another thermoplastic resin film containing a fine inorganic powder; after cooling, the laminate is heated to a stretching temperature, then stretched transversely on a tenter and heat set; following further cooling, a surface of the heat set assembly is optionally subjected to corona discharge treatment, thereby producing a sheet of synthetic paper. This related process is described in U.S. Pat. No. 4,318,950 and FIG. 2 accompanying this patent shows an apparatus for producing such synthetic paper.
There is also a related process for producing a low-temperature heat-sealed film suitable for use in packages such as cigarette cartons and candy boxes. The process comprises first stretching polypropylene film with a heat-melted film of a propylene-ethylene copolymer having a lower melting point than the polypropylene in a machine direction after cooling, heating the laminate and then stretching the laminate on a tenter in a transverse direction at a temperature lower than the melting point of the polypropylene but higher than the melting point of the copolymer and thereafter heat treating the biaxially stretched film laminate.
A related process for producing a pearly film comprises first stretching a resin film containing a fine inorganic powder in a machine direction on a series of rolls, then stretching the film in a transverse direction on a tenter and heat setting the biaxially stretched film (see U.S. Pat. No. 3,765,999).
In these methods of producing synthetic papers, low-temperature heat-sealed laminate films and pearly films, the temperature of the annealing hot air oven is set to be slightly higher than that of the stretching hot air oven in the tentering oven. The stretching oven is sometimes an integral part of the heat treatment oven and, as shown in FIG. 1 (as shown in FIG. 2 of Examined Japanese Patent Publication No. 9130/1992), a resin film 1 being supplied in these ovens is heated by a plurality of plenum ducts 2, 2, . . . (i.e., 9 plenums) which have a length substantially the same as the transverse width of the resin film 1 and which are positioned both above and below the resin film such that hot air as supplied from the ducts 2, 2, . . . is passed through discharge ports 3, 3, . . . to be blown against the resin film 1.
The hot air is supplied from conduits 5, 5, . . . to the plenum ducts and then passed through the discharge ports 3, 3, . . . to be blown against the resin film 1 to heat it.
As shown in FIG. 2 (corresponding to FIG. 9 in Examined Japanese Patent Publication No. 9130/1992), the stretching hot air oven 10 also contains two rows of plenum ducts 2, the upper row consisting of 4 plenum ducts and the lower row consisting of 3 plenum ducts. The plenum ducts in the annealing hot air oven are interconnected with conduits 5 via holes 6 in the back surfaces of individual plenum ducts, the plenum ducts in the stretching hot air oven are interconnected with another group of conduits 5 via holes 6 in the back surfaces of those plenum-ducts. The individual conduits 5 are supplied with air that has been heated after aspiration with a blower. The temperature of the heated air (hot air) is typically about 170.degree. C. in the stretching hot air oven 10 and about 175.degree. C. in the annealing hot air oven 11 if the film is made of polypropylene.
The stretched film emerging from the stretching and heating zones enters a cooling zone, where it is cooled with air blown from plenum ducts (as shown in Examined Japanese Patent Publication No. 63498/1991).
The resin film which was typically at 160.degree. C. in the annealing oven is cooled in the cooling furnace such that it leaves the exit thereof at 30.degree. to 50.degree. C.
In the tentering oven, the film is heated while at the same time is stretched with the tenter by a stretch ratio of 4 to 12 in a transverse direction (TD) which is perpendicular to a machine direction (MD), or the flow of the film.
As shown in Examined Japanese Patent Publication No. 18513/1975, the tenter comprises principally clip bodies, clip levers and liners provided on linked chains, as well as rails composed of chain directing rail members that progressively depart away from each other and chain drive means (a motor).
A clip used in a tenter-type stretching machine is shown in FIGS. 3(a), 3(b) and comprises linked chains 12 and 13 carrying a clip 14 thereon. The clip 14 is basically composed of a clip body 15, a clip lever 16 and liner 17 carried on the clip body 15. At the entrance of the tentering oven, the clip 14 clips a film 1 to be stretched transversely. The thus clipped film is thermally stretched and heat set as it is advanced through the oven at a controlled temperature and at the exit of the tentering oven, the stretched film is released from the clip lever 16 such that it is disengaged from the clip 14 and transferred to wind-up and other subsequent steps.
In order to achieve the intended stretch ratio in TD, a multiple of rail members 18 (FIG. 4) (usually available in lengths of 1.5 meters and 3 meters) are joined together. In areas where changes in width are frequent, rail members 1.5 m long are used; on the other hand, 3-m long rail members are used in areas where the stretching pattern is fixed.
As shown in FIGS. 6, 7(a) and 7(b), an oil 22 used to lubricate chains 12 and 13 drips down through a joint 19 (see FIG. 4) between rail members 18. In order to prevent the dripping oil from fouling the film 1, an oil pan 20 was placed below the individual joints 19 as shown in FIG. 4. It was then necessary that the discharge port 2a of a plenum duct located below each oil pan be provided with a blind patch 50 (see FIG. 4) in order to prevent the oil into the entrance of the duct while ensuring that the hot air coming through the discharge port will not impinge against the oil pan to create a turbulent flow that will scatter the oil drops.
However, it has been found that if both ends of the discharge port are provided with a blind patch 50, the ends of the film are heated less than the central portion and the uniformity in the thickness profile of the stretched film in the direction of its width (in transverse direction) is reduced--the deviation is .+-.4.0 .mu.m on an 80 .mu.m thick film. It has also been found that even if the blind patch 50 is fitted, oil drops 22' will adhere to the stretched film when 45 to 60 days have passed since the beginning of the stretching operation. Stated more specifically, the oil leaking from the upper holder flows down the walls to collect in drops 22' (see FIG. 6) under the rails and the supplied hot air will cause such oil drops to fly about within the tentering oven, thereby fouling the film being stretched.
Therefore, if oil drops are detected on the surface of the stretched film 1, the operation of the molding machine has to be stopped so that the tenter can be cleaned. The interval of tenter cleaning operations ranges from 1.5 to 2 months and, even if the molding machine is restatted after the cleaning of the tenter, it .usually takes about 12 hours to establish a steady-state operation.